1. Field of the Invention
The invention relates to an electromagnetic wave modulator comprising a quantum well semiconductor structure, working notably in the 8-13 .mu.m range of wavelengths.
2. Description of the Prior Art
For many years now, the development of the techniques for growing semiconductor structures has enabled the making of artificial materials with novel quantum properties. Indeed, the association of different semiconductors in thin layers results in discontinuities of the conduction band and of the valence band all along the axis of growth. These discontinuities of the potential energy of the electrons in the conduction band and of the holes in the valence band have been extensively studied and exploited in high mobility transistors, for example. By associating two semiconductor materials, it is thus possible to form two steps of potential energy having opposite directions, this result being obtained on a distance that is shorter than the de Broglie wavelength of the electron or of the hole. In such a system, which is also called a quantum well system, the electron states are no longer distributed in a continuum but in subbands, each being associated with a quantum number and an energy level. When the well is doped, it is possible to induce electron transitions from one subband to another by means of a properly chosen electromagnetic wave with a wavelength .lambda.: these are the inter-subband transitions illustrated in FIG. 1. The dipole associated with these transitions has a size comparable to the width L of the quantum well (giving about 0.18 L) and the corresponding state density has the shape of a Dirac function. The absorption from the fundamental level, referenced .vertline.0&gt; towards the first excited level referenced 1.vertline.1&gt; is therefore resonant and relatively intense in a symmetrical well.
In the GaAs/GaAlAs type III/V semiconductor systems, this absorption is about 0.3% when these semiconductors are doped with 10.sup.12 electrons per cm.sup.2 and illuminated at the Brewster angle. Many approaches have already been considered to obtain the modulation of this electromagnetic wave by using the inter-subband transitions within doped multiple quantum well structures.
A first approach pertains to the modification of the states and energies E.sub.i of a multiple quantum well structure by the application of an electrical field. The energy difference between levels is thus disturbed and, consequently, the resonance photon energies: E.sub.j -E.sub.i =hc/.lambda..sub.ij (with h being the Planck's constant, c the velocity of light, .lambda..sub.ij the wavelength) are modified. This electrooptical effect is low in the symmetrical structures for it is a third-order non-linear susceptibility that is brought into play. In an asymmetrical structure, this effect is linear and more effective for it is then essentially a second-order susceptibility that comes into play. The absorption peaks get shifted under the effect of the electrical field applied. This is the Clark effect, which was clearly demonstrated in 1990 ("Tunable Infrared Modulator And Switch Using Stark Shift In Step Quantum Wells", R. P. G. Karunasini, Y. J. Mii, K. L. Wang, IEEE Electron Device Letters, Vol. 11, No. 5). Taking a given wavelength, and amplitude, a phase modulator is thus defined by simple translation of the dispersal of real and imaginary indices associated with the inter-subband transition, this being achieved under the effect of an electrical field.
A second approach consists in the making of a charge transfer modulator ("Tunnelling Assisted Modulation of the Inter-Subband Absorption in Double Quantum Wells", M. Vodjdani, D. Vinter, V. Berger, in Applied Physics Letters, vol. 59, No. 5). This type of device seeks the modification, by electrical means, of the concentration N.sub.S of carriers per unit of area. To do this, two weakly coupled wells are used. At negative voltage, only the large well is populated and the inter-subband transition is relative to this well. By applying a positive voltage, the carriers are transferred, by tunnel passage, into the narrowest well as illustrated in FIG. 2. Since the fundamental level of this well is populated, it is possible to measure an inter-subband absorption associated with this well, hence one with a shorter wavelength. Thus, a two-color modulator is obtained.